body {
background-color: #b5b5b5;
}

#loginform {
background-color: yellow;
width: 70%;
height: 50%;
position: absolute;
left: 50%;
top: 50%;
margin-left: -35%; /* (100 - width) / 2 */
margin-top: -25%; /* (100 -  height) / 2 */
}